1. Field of the Invention
The present invention relates generally to diversity communication systems, and more specifically to a time division multiple access/time division duplex (TDMA/TDD) communication system that utilises the transmit diversity of the system by the transmission of bursts from best antennas selected according to the strengths of received signals.
2. Description of the Related Art
Digital radio communication systems of TDMA/TDD mode are currently employed for mobile cellular networks such as the Personal Handyphone Systems (PHS) and are standardised as the Digital European Cordless Telecommunication (DECT). Since this mode of transmission employs only one frequency for both transmission and reception, the mobile terminal is provided with a transmit/receive switch, instead of a costly antenna duplexer.
It is known that because of the high-speed mode switching between transmission and reception relative to the rate of occurrence of multipath fading, there is a high degree of correlation between a downlink propagation path and an uplink propagation path. If a cell-site base station uses a number of space diversity antennas, a best antenna for the reception of an uplink signal from a given mobile terminal is also the best for the transmission of a downlink signal to the given mobile terminal. This mode of communication takes advantage of diversity reception of signals and is called “transmit/receive diversity”. One example of known transmit/receive diversity mode communication systems is shown and described in Japanese Laid-Open Patent Application No. 10-51365.
In the current TDMA/TDD system, it is required that downlink RF bursts transmitted to mobile terminals during a first half interval of a frame must be in a one-to-one correspondence to uplink RF bursts received from the mobile terminals during a second half interval of a frame and that each burst must be sent and received on time slots assigned to each mobile terminal. Further, RF bursts must be sufficiently spaced apart by “guard times” to accommodate possible variability of propagation delays due to different propagation path lengths. Additionally, each RF burst must include header information such as a preamble and a unique word for establishing bit and frame synchronisation.
It is thus inefficient in terms of channel utilization. This is particularly true of the transmit/receive diversity system in which a number of downlink RF bursts are transmitted in sequence from one selected antenna.